Work is ongoing in 3GPP, 3rd Generation Partnership Project, and with some operators to specify and investigate the possibility to provide home and/or small area coverage for a limited number of users using a small base station, commonly called a Femto NodeB for WCDMA, Wideband Code Division Multiple Access, or Femto eNodeB (E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) NodeB) for LTE, Long Term Evolution. Other commonly used names are HNB (Home NodeB) for WCDMA and HeNB (Home eNodeB) for LTE. The femto solutions based on WCDMA technology are called WCDMA Femto solutions and the solutions based on LTE technology are called LTE Femto solutions. Similar solutions can also be deployed for other radio technologies, like GSM. In this application, the name Femto NodeB is used as an exemplary name for the small base station and should not be read to limit the scope only to WCDMA Femto solutions. In the same way, the term Macro NodeB is used as an exemplary name for any traditional base station using any technology. The Femto NodeB would provide normal LTE/WCDMA/GSM coverage for the end users, a so called femto cell, and would be connected to the mobile operator's network using some kind of IP based transmission. One example is to use fixed broadband access (e.g. xDSL or Cable) to connect the Femto node to the mobile operator's network. Another alternative would be to use Mobile Broadband access, for example, High Speed Downlink Packet Access, HSDPA, and Enhanced Uplink.
There are several ways to use the available radio frequencies between the femto layer and the traditional cellular deployment layer, referred to as ‘macro’ layer even though it may comprise both macro, micro and/or pico cells. There are three main Channel Deployment Scenarios (CDS) that are relevant, namely:                CDS1: One frequency is used by both the Femto NodeBs and the Macro NodeBs. This CDS has some severe interference problems that will make it hard to work.        CDS2: One dedicated frequency is used only by the Femto NodeBs and (at least) another frequency is used only by the Macro NodeBs. This CDS is the preferred one if only technical arguments are taken into account. However, it is very unlikely that operators are willing to dedicate whole frequencies for the femto layer, mostly due to economical reasons.        CDS3: One frequency used both by the Femto NodeBs and the Macro NodeBs and (at least) another frequency used only by the Macro NodeBs.        
CDS3 is expected to be a common way for deployments. This deployment scenario may as well as CDS1 cause different types of interference in certain scenarios, both between the femto and the macro layer and internally in the femto layer as it is assumed that all the Femto NodeBs belonging to one operator are using the same carrier/frequency.
A number (a few) Primary Scrambling Codes (PSC) is allocated to the femto layer, e.g. ten. These PSCs are configured in the cell neighbor lists on the macro layer, and signaled to macro layer user equipments, UEs, for their idle mode cell selection. One of these PSCs is automatically assigned to each Femto NodeB at start up.
A number (a few) Location Areas (LA) is also allocated to the femto layer, e.g. ten. One of these is automatically assigned to each Femto NodeB at start up.
A Femto NodeB scans and reports found cells (both macro and femto cells) to a Radio Network Controller, RNC. The RNC may be a standalone entity or combined with the Femto NodeB. The reported data includes for each detected cell:
1. PSC,
2. frequency, i.e. UTRA Absolute Radio Frequency Channel Number, UARFCN,
3. relevant parts of system information (e.g. Location area identifier, LAI, Routing area code RAC, Cell-ID, primary common pilot channel CPICH transmission power) and
4. signal strength measurement report.
The RNC builds the neighbor list for a Femto NodeB, and includes the heard macro cells and the allocated femto PSCs in the neighbor list. The list is signaled both to UEs camping on the femto cell of Femto NodeB as broadcasted system information for use in cell selection/reselection and to UEs connected to the Femto NodeB as dedicated signaling in the form of Monitored Set for use in measurement reporting for possible handover.
Femto NodeB ‘access control’ may for example be based on a forbidden LAI list in the UE (i.e. LAI list in UE updated with the use of Location Update Reject).
By using frequency deployment scenario CDS3, as well as by using CDS1, it is possible that different types of interference are created when a UE camped on a femto cell of a first frequency moving towards a neighboring cell.
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